The present invention relates to a porous structural unit used for an acoustic absorption material or a heat insulation material. More particularly, it relates to a porous structural unit having a porous layer wherein the specific gravity is continuously changed in the direction of the thickness of the layer or of the surface of the layer, and a method of preparing the same.
There have been used as an acoustic absorption material or a heat insulation material a porous material such as glass wool, rock wool, urethane foam and so on. The porous material such as urethane foam, steel wool and so on have been used as a filter for cleaning air. The porous material is widely used for the purpose of sound deadening, heat insulating and air cleaning such as a vacuum cleaner, a cooling/warming air conditioner, an air cleaner and so on. There is a strong demand from manufacturers that the porous material be low cost and high in performance as well as the minimum restrictive conditions (e.g. the shape be small). Generally, an acoustic absorption material or an heat insulation material is so constructed that an inner lining is attached to a structural body made of a non-gas-permeable material. The structural body has functions to shield sounds and to form a part of a flow passage for air. A filter unit is formed by assembling a porous material in a frame of a non-gas-permeable material. The frame has a sealing effect so as not to cause leakage of a fluid from the periphery of the porous material in the filter unit. The porous material and the non-gas-permeable material can be prepared separately and then assembled. Or, the porous material is formed by using a foaming material and then, a part of the surface of the porous material is processed to have non-gas-permeability.
The above-mentioned porous structural unit and the method of preparing of the unit are disclosed in, for example, Japanese Unexamined Patent Publication 113172/1978 "vacuum cleaner", Japanese Examined Patent Publication 52132/1983 "indoor unit of air conditioner", Japanese Unexamined Patent Publication 1045/1971 "multi-cellular thermoplastic material, a complex consisting of said material and a thermoplastic sheet layer, and a method of preparing the same" and Japanese Unexamined Patent Publication 19654/1973"process for forming a laminated soft outer layer".
In the conventional method of preparing a porous structural unit, there were problems that a combination of the porous layer and a layer (e.g. a non-gas-permeable layer) having a higher specific gravity required much cost; the method of using a foaming material required additional cost because a side surface had to be made non-gas-permeable, and it was difficult to obtain a complicated configuration.
Further, the conventional porous structural unit had problems that since it had a simple shape wherein the porous layer having a uniform specific gravity and a layer (e.g. a non-gas-permeable layer) having a higher specific gravity were combined, it was difficult to give the optimum distribution of specific gravity and give the optimum shape in order to further increase the performance, and it was difficult to obtain good sound absorption characteristic and good heat insulation characteristic.